Chemical Kinetics and Aqueous Degradation Pathways of a New Class of Synthetic Ozonide Antimalarials CHRISTINE S. PERRY, 1 SUSAN A. CHARMAN, 1 RICHARD J. PRANKERD, 1 FRANCIS C.K. CHIU, 1 YUXIANG DONG, 2 JONATHAN L. VENNERSTROM, 2 WILLIAM N. CHARMAN 1 1 Centre for Drug Candidate Optimisation, Victorian College of Pharmacy, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia 2 College of Pharmacy, University of Nebraska Medical Centre, Omaha, Nebraska 68198 Received 22 August 2005; revised 11 November 2005; accepted 16 November 2005 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.20568 ABSTRACT: Chemical stability of a new class of ozonide (1,2,4 trioxolanes) antimalarial compounds was investigated. The effects of pH, ionic strength, dielectric constant and cyclodextrin-complexation on the chemical stability and degradation product formation of selected compounds were examined. The mechanism of degradation in aqueous solution was probed using 18 O-labelled water and kinetic solvent isotope effect studies. The effect of stereochemistry was investigated using selected pairs of stereoisomers. The degradation of the ozonides in aqueous solution followed apparent first-order kinetics, with no effect of ionic strength and no indication of any direct involvement of water in the degradation mechanism. All major degradation products were identified and mass balance was confirmed. Stereochemistry had a significant effect on degradation rate; trans isomers degrading approximately four-fold faster than the corresponding cis isomers. The degradation rates were essentially independent of pH above pH 2; however, an additional specific acid catalysed pathway was dominant below pH 2. Solvent dielectric constant had a significant effect on the degradation rate. It is proposed that the degradation observed in aqueous solution occurred through a concerted heterolytic scission of the central ozonide ring, with chemical substituents on the cyclohexyl ring having only a minor influence on degradation rate. ß 2006 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:737– 747, 2006 Keywords: ozonide; antimalarial; 1,2,4 trioxolane; degradation; peroxide; chemical stability; kinetics; mass spectrometry; cyclodextrins; antiinfectives INTRODUCTION Malaria infects between 300 and 500 million people each year, with the majority of cases being children and pregnant women. 1 Parasite resistance to existing antimalarials such as chloroquine, sulfadoxine and pyrimethamine is increasing throughout the developing world and is driving the urgent need for new antimalarial therapies. 2 To date, there have been no reports of clinical resistance to endoperoxide antimalarials such as artemisinin. The antimalarial activity of peroxides has been attributed to the labile nature of the peroxide bond. 3–5 Rupture of this bond, which occurs during the parasite’s digestion of haemo- globin, generates carbon-centred free radicals which are postulated to irreversibly interact with proteins essential for parasite survival. 6,7 The lack of clinical resistance to these compounds appears to be a consequence of the nonspecific and reactive nature of the radical species generated. 8 However, despite their clinical efficacy, the supply and economics of the semisynthetic artemisinins have limited their utility; and the inherent issues of purity, cost and poor biopharmaceutical properties JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 95, NO. 4, APRIL 2006 737 Correspondence to: William N. Charman (Telephone: þ61 3 9903 9519; Fax: þ61 3 9903 9560; E-mail: bill.charman@vcp.monash.edu.au) Journal of Pharmaceutical Sciences, Vol. 95, 737–747 (2006) ß 2006 Wiley-Liss, Inc. and the American Pharmacists Association